1) Field of the Invention
The present invention relates to a belt member, a belt driving unit, and an image forming apparatus.
2) Description of the Related Art
Conventionally, annular belt members, which are endlessly moved while being stretched by a plurality of stretch members, have been used in various fields. For example, in an electrophotographic type image forming apparatus, an annular belt member is used as a latent image carrier, such as a photosensitive element, or an intermediate transfer unit. In such electrophotographic type image forming apparatus, an image is formed in a following process. First, a latent image carrier is exposed, and an electrostatic latent image of a scanned image is formed on the latent image carrier. To the electrostatic latent image, a developer, such as a toner negatively-charged or negatively-charged, is applied. Thus, a toner image is formed. The toner image is then transferred onto a recording medium, such as a transfer sheet, directly from the latent image carrier, or through an intermediate transfer unit. The toner image transferred is then fixed to the recording medium by a process, such as heating. The annular belt member is used as the latent image carrier or the intermediate transfer unit that are used in such an image forming process in the image forming apparatus.
In some of the image forming apparatuses, timing for various actions such as image forming and sheet feeding, are determined based a reference mark that is provided on the annular belt. Such timing is determined by detecting the reference mark. The reference mark is provided at a predetermined position in a direction of circulation of the annular belt. For example, in an image forming apparatus disclosed in Japanese Patent Application Laid-open No. H11-15297, a reflective reference mark is provided at a predetermined position in an intermediate transfer belt, and is detected by a reflective photosensor. The image forming apparatus determines exposure timing for a photosensitive element based on timing at which the reflective reference mark is detected.
In another image forming apparatus disclosed in, for example, Japanese Patent Application Laid-open No. H9-114348, more than one mark is provided at predetermined intervals on a belt member in a direction of circulation, and a driving speed of the belt member is controlled based on time intervals at which the marks are detected. A reflective photosensor in such image forming apparatus detects light reflective marks, and based on fluctuations in time intervals at which the light reflective marks are detected, fluctuations in the running speed are detected. When the fluctuations are detected in the running speed, the running speed is adjusted to be a target speed. Thus, fluctuation of the running speed is suppressed.
However, the reference marks and the marks (hereinafter generally, “mark”) are gradually stained as the belt member is driven by a belt driving unit. If the mark is stained so badly that a sensor cannot detect the mark, various malfunctions may occur in the image forming apparatus. The malfunctions include inappropriate timing determination for forming an image and erroneous detection of the fluctuation in the running speed. Such a problem is more likely to occur in an image forming apparatus that use a colored developer, such as a color toner, due to adhesion of the colored developer.
The inventors of the present invention have been developing a novel belt member that has a protection layer for the mark. The protection layer is translucent, and is provided on the mark to protect the mark. Generally, surface treatment with chemicals or by polishing is required to provide a good light reflection property to the mark. By the surface treatment, the mark becomes more likely to let stains adhere thereon. The protection layer in the belt member that is under development by the inventors does not require the surface processing. Therefore, adhesion of stain over the mark is prevented, thereby preventing occurrence of the malfunctions described above that are originated from the stain on the mark.
However, the protection layer is gradually damaged with scratches being made with use of the belt member. As the protection layer is damaged, a translucence of the protection layer is degraded. As a result, the mark cannot be detected properly. Specifically, the belt driving unit that drives a belt member is provided with a cleaning member that cleans a stain of toner adhered to a surface of the belt member. The stain is scraped off by sliding the cleaning member, such as a plate scraper and a brush, on the surface. While repeating such a process by the cleaning member, the protection layer is gradually damaged.